The present invention relates to polymer films and a polymer membrane having an improved mechanical property profile produced therefrom, to a process for producing them and to their use.
Owing to its excellent chemical, thermal arid mechanical properties, the acid-doped polymer membrane described below can be used in a wide variety of applications and is suitable, in particular, as polymer electrolyte membrane (PEM) in PEM fuel cells.
Acid-doped polyazole membranes for use in PEM fuel cells are known. The basic polyazole membranes are doped with concentrated phosphoric acid or sulfuric acid and act as proton conductors and separators in polymer electrolyte membrane fuel cells (PEM fuel cells).
For this application, electrodes coated with catalyst are applied to both sides of the acid-doped polyazole membranes to form a membrane-electrode unit (MEE). A plurality of such membrane electrode units are then connected in series together with bipolar plates and form the fuel cell stack.
As a result of the series construction, the cell voltage and power of the stack depends on the number of membrane-electrode units. Furthermore, failure of a single one of these membrane-electrode units results in a break in the circuit and thus failure of the entire fuel cell. For this reason, extraordinarily high quality demands are made on the mechanical stability of all components. The thin, usually 100 μm thick polymer membrane in particular is frequently regarded as the weakest link in this chain. The membrane performs two essential tasks. Firstly, it has to have a high proton conductivity in order to be able to conduct the protons formed in the oxidation of a hydrogen-rich fuel at the anode to the cathode. There, reduction with oxygen, preferably from air, then takes place with formation of water. Secondly, the membrane functions as a separator and should have a very low permeability to the fuels present. In particular, when hydrogen and oxygen are used, mixing of the two gases has to be prevented. For this reason, the polymer membrane should not fail in operation, even at high temperatures.
The mechanical stability of the thin (usually <0.2 mm) polymer film is reduced by the doping with acid to generate a high proton conductivity. To be able to withstand the stressing of the cells at operating temperatures of >100° C. over the long term, extremely resistant polymers have to be used.
Due to the excellent properties of the polyazole polymer, polymer electrolyte membranes based on polyazoles, converted into membrane-electrode units (MEE), can be used in fuel cells at long-term operating temperatures above 100° C., in particular above 120° C. This high long-term operating temperature allows the activity of the catalysts based on noble metals which are present in the membrane-electrode unit (MEE) to be increased. Particularly when using reformer products produced from hydrocarbons, significant amounts of carbon monoxide are present in the reformer gas and these usually have to be removed by means of a costly gas work-up or gas purification. The ability to increase the operating temperature enables significantly higher concentrations of CO impurities to be tolerated over the long term.
The use of polymer electrolyte membranes based on polyazole polymers allows, firstly, the costly gas work-up or gas purification to be partly omitted and, secondly, the amount of catalyst in the membrane-electrode unit to be reduced, They are indispensable prerequisites for wide use of PEM fuel cells, since otherwise the costs of a PEM fuel cells system are too high.
The acid-doped polyazole-based polymer membranes known hitherto display a favorable property profile. However, owing to the applications sought for PEM fuel cells, especially in the automobile and stationary sector, these need to be improved overall.
Thus, the polyazole-based polymer membranes known hitherto display mechanical properties after doping with acid which are still unsatisfactory for the above application. This mechanical instability is shown by a low modulus of elasticity, a low ultimate tensile strength and a low fracture toughness.